1. Field of the Invention
The present invention relates generally to a vehicle service tool, and more particularly to caliper, which is applicable to, but not limited to, a brake disc for measuring the thickness thereof.
2. Description of the Related Art
It is known that a disc brake system includes a brake master cylinder and a brake wheel cylinder. When a driver pedals the brake pedal, via the fluid in the brake master cylinder, the pressure is transmitted to the piston of the brake wheel cylinder on the brake caliper for driving the brake linings to tightly clamp the brake disc. In this case, the rotational speed of the wheel is lowered down under the frictional force against the brake disc so as to achieve the object of braking.
However, the braking effect is achieved by means of the friction between the brake linings and the brake disc. Therefore, the wear of the contact faces of the brake linings and the brake disc is inevitable. With respect to the brake linings, the wear is relatively obvious and thus it is necessary to periodically replace the brake linings. With respect to the brake disc, the wear is concentrated on a part of the brake disc that contact with brake linings. Referring to FIG. 3, the brake disc 50 is only partially worn to form two annular grooves 51 on two faces of the brake disc 50 respectively. It is known that the travel of the brake wheel cylinder is fixed. Therefore, in the case that the brake disc 50 is over-worn and the thickness between the bottom walls of the annular grooves 51 is too thin, the braking performance will be directly affected to threaten the safety in driving.
According to the conventional technique, the thickness of the brake disc is generally measured by means of a caliper ruler or a micrometer caliper. With respect to the caliper ruler, the measurement jaws can simply attach to a complete plane face so that the caliper ruler can be only used to measure a brand-new brake disc, which has not been used yet. As to a used and worn brake disc with the annular grooves on two faces, the caliper ruler can be hardly used to measure the thickness of the brake disc. With respect to the micrometer caliper, it can be used to measure the thickness of both a brand-new brake disc and a worn brake disc with the annular grooves. However, the measurement value shown by the micrometer caliper only indicates the distance between the shaft and the anvil. As a result, once the distance between the shaft and anvil changes, the shown value will synchronously change. Accordingly, it is necessary for a user to read the brake disc thickness value of the micrometer caliper still connected to the wheel hub in a narrow space and then take off the micrometer from the brake disc. Therefore, it is quite inconvenient to use the micrometer caliper.